In recent years, the aging of population has progressed, and there has been a growing number of hearing-impaired people due to aging. In order to maintain the quality of life of the elderly, it is important for them to be able to hear sounds that they cannot hear as they grow older, especially words. Thus, it is considered that the need for hearing aids will increase more and more.
On one hand, a deterioration in hearing of the hearing-impaired people differs from person to person. Accordingly, when a hearing-impaired person directly wears an off-the-rack hearing aid, the hearing aid is not suitable for the specific hearing impairment of such a person. In other words, it is necessary to measure aural characteristics of buyers when selling hearing aids, and to set each of the hearing aids according to a symptom of a corresponding one of the buyers.
In the currently most common aural characteristics test, a pure tone is produced by using an audiometer, and the minimum sound pressure level (hearing level) at which subjects can hear is measured. Generally, the lowest audibility level (hereinafter, referred to as a hearing level) is measured for the respective sinusoidal waves (pure tones) of 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz. In addition, if necessary, the same measurement may be performed for the respective sinusoidal waves of 750 Hz, 1500 Hz, 3000 Hz, and 6000 Hz. A unit of hearing level is dB (decibel). A sound pressure level at which people having a normal hearing ability can hear is 0 dB, a hearing ability diminishes as a value of a hearing level increases. A graph on which a hearing level at each of frequencies is plotted is called an audiogram, and is widely used to set the hearing aids. FIG. 1 shows an example of an audiogram.
FIG. 2 is a block diagram showing a common hearing assistance adjustment system 900. The hearing assistance adjustment system 900 shown in FIG. 2 causes a hearing-impaired person to hear, through headphones, a pure tone produced by an audiometer 901. Then, a measuring person operates the audiometer 901 when the hearing-impaired person hears the tone for measurement, so as to measure a hearing level of the hearing-impaired person.
After the audiometer 901 completes the measurement of the hearing level, in a hearing assistance adjustment device 901, a hearing level obtaining unit 911 obtains a hearing level at each of frequencies measured by the audiometer 901, as shown in FIG. 3. Next, an amplification amount calculating unit 912 calculates, based on the obtained hearing level at each frequency, an amplification amount for compensating a deterioration in hearing of the hearing-impaired person.
An amplification amount display unit 913 notifies a person in charge of adjustment of the calculated amplification amount. Furthermore, a hearing aid setting unit 914 sets the calculated amplification amount to an internal parameter of a hearing aid 903.
The adjustment of the hearing aid 903 for compensation (amplification) according to a hearing level of an individual is completed through the above procedure.
Moreover, Patent Literature 1 has proposed, in conventional hearing assistance adjustment processing, a method for substituting an amplification amount setting process having specific patterns for an amplification amount setting process which is performed in detail for each frequency, by classifying a general shape shown in audiograms into some predetermined values and combining the predetermined values with an overall hearing level.
On the other hand, it is said that frequency resolution of the hearing-impaired people has been reduced in addition to reduced hearing sensitivity. Here, the frequency resolution is a subject's ability to differentiate between two tones having different frequencies. Normal-hearing people can differentiate between two tones having adjacent frequencies such as 1 kHz and 1.2 kHz. However, the hearing-impaired people whose frequency resolution has been reduced cannot differentiate between the two tones.
Furthermore, in recent years, a model in which auditory filters are used has been proposed as a model for representing a frequency-analysis mechanism for human auditory. This model represents the frequency-analysis mechanism for human inner ears with an aggregate of band filters (auditory filters) which divide the mechanism into frequency bands. When a width of an auditory filter is large, it is considered that frequency resolution has been reduced.
Large reduction in the frequency resolution grows an influence of masking between frequency band components, particularly masking high-frequency components by low-frequency components (upward masking). Especially in speech, a vowel having a main component in low frequencies has a large amount of energy. The vowel having the large amount of energy masks a consonant having a main component in high frequencies. This causes a problem such as reduction in ability to hear words or significant reduction in speech discrimination ability in noise.
The problem caused by the deterioration in frequency resolution is a phenomenon based on a principle different from the reduced hearing sensitivity. Therefore, amplification of sound by the hearing aids cannot solve the problem, and the problem has been a major hurdle for the hearing-impaired people in terms of the hearing of words.
Examples of hearing assistance processing which is intended to increase clarity of audio input signals by reducing the masking between frequency bands include dichotic hearing assistance with which an input signal is divided on the frequency axis to be assigned to respective left and right ears. Past study examples have proposed, in the dichotic hearing assistance, a method for dividing an input audio signal by a frequency in a region where auditory filters are placed, and assigning an audio signal having a frequency lower than a crossover frequency to one of the ears and an audio signal having a frequency higher than the crossover frequency to the other ear. It has been reported that this method can increase clarity of speech (Non Patent Literature 1). FIG. 4 shows a structure for deciding setting of the hearing assistance method. In FIG. 4, an auditory filter measuring unit 951 measures an auditory filter. Moreover, it is decided by which frequency a tone region is divided into a high-tone region and a low-tone region, according to a value obtained by a frequency resolution calculating unit 952. Patent Literature 2 has proposed a speeding-up method concerning the auditory filter measuring unit 951.
Nevertheless, it has been reported that the dichotic hearing assistance cannot increase the clarity for all of the hearing-impaired people, and did not produce an effect on some hearing-impaired people (Non Patent Literature 1). Thus, when the dichotic hearing assistance is applied to hearing aids, it is necessary to determine whether or not to recommend the dichotic hearing assistance to a hearing-impaired person for whom adjustment of the hearing aids is made. However, measuring an auditory filter using the above method allows only prediction of a part of a setting value of the dichotic hearing assistance. In other words, currently there is no established method as a suitability determining method for the dichotic hearing assistance.